Slant Axis fallbacks revision

[felipesanches]

Fonts to be onboarded that have this axis:

• Update Nunito Sans to VF format google/fonts#4286
• Upgrade Cairo with slant axis google/fonts#4059
• Upgrade Gluten with slant axis google/fonts#3789

[felipesanches]

@RosaWagner, at google/fonts#4286 (comment) you said that the Nunito Sans update needs registration of a slant axis (and also an xheight one). But we do have a 'slnt' axis!
https://github.com/googlefonts/axisregistry/blob/main/Lib/axisregistry/data/slant.textproto

[RosaWagner]

The slant axis has empty fallback values, therefore italic styles can’t be generated by the API. It would therefore create regression if we onboard an upgrade with a slant axis instead of 2 VF Roman/Italic

[RosaWagner]

But we can do as Nunito and onboard it as 2 VF but since it requires another axis and (I think) change of politic around the mvar table, we may as well wait.

[yanone]

This can be closed, can it not?

[vv-monsalve]

Slant Axis discussion is open in #295

[felipesanches]

Thanks for the cross-referencing, @vv-monsalve

[felipesanches]

This can be closed, can it not?

This issue can only be closed once the axis is properly added, or if we formaly reject the proposal.

In the case of rejections, I think it would be useful to keep a log file at this repo describing the formal reasoning for each rejection.

[RosaWagner]

Other discussion open there: google/fonts#2606